Fuel cells are classified into several types according to electrolytes or electrodes. Typical types are alkaline types, phosphoric acid types, molten carbonate types, solid electrolyte types and polymer electrolyte types. In particular, polymer electrolyte fuel cells that can operate at temperatures ranging from low temperatures (about −40° C.) to about 120° C. attract attention and are progressively developed and practically used as power sources for low pollution automobiles. The polymer electrolyte fuel cells are expected to be used as automobile drive sources or stationary power sources. The use in these applications requires long-term durability.
The polymer electrolyte fuel cell has a solid polymer electrolyte sandwiched between an anode and a cathode. A fuel is fed to the anode, and oxygen or air is supplied to the cathode, whereby oxygen is reduced at the cathode to produce electricity. The fuel is usually hydrogen or methanol.
To increase the reaction rate in fuel cells and enhance the energy conversion efficiency, a layer containing a catalyst (hereinafter, also the fuel cell catalyst layer) is conventionally provided on the surface of a cathode (an air electrode) or an anode (a fuel electrode) of fuel cells.
Here, noble metals are generally used as the catalysts. Of the noble metals, platinum that is stable at high potential and has high catalytic activity is most frequently used. However, since platinum is expensive and exists in a limited amount, alternative catalysts have been desired. Further, the noble metals used on a cathode surface are often dissolved in an acidic atmosphere and are not suited in applications requiring long-term durability. Accordingly, it has been strongly demanded that catalysts are developed which are not corroded in an acidic atmosphere and have excellent durability and high oxygen reducing activity, and further is inexpensive in inexpensive in terms of electricity generation cost.
Further, the noble metals used on a cathode surface are often dissolved in an acidic atmosphere and are not suited in applications requiring long-term durability. Accordingly, it has been strongly demanded that catalysts are developed which are not corroded in an acidic atmosphere and have excellent durability and high oxygen reducing activity.
Materials containing nonmetals such as carbon, nitrogen and boron capture attention as alternative catalysts to platinum. The materials containing these nonmetals are inexpensive compared to noble metals such as platinum and are abundant.
Non-patent Document 1 reports that zirconium-based ZrOxN compounds show oxygen reducing activity.
Patent Document 1 discloses, as platinum-alternative materials, oxygen-reducing electrode materials containing a nitride of one or more elements selected from Groups 4, 5 and 14 in the long periodic table.
However, the materials containing these nonmetals do not provide sufficient oxygen reducing activity for practical use as catalysts.
Patent document 2 considers the possibility of providing a perovskite structure-having oxides which contain two or more kinds of metals as a platinum-alternative catalyst. As is clear from Example of this document, however, such a catalyst has limited effect of serving as a carrier to aid the platinum, and thus does not have a sufficient activity.